The present invention relates to high frequency signal processing. Specifically, a method and apparatus are described which provide for group delay equalization of network filters having a non-linear group delay.
In the field of telecommunications where analog and digital signal processing are necessary, filters, such as bandpass and lowpass filters, or delay lines are needed at various points in the signal processing network to condition a signal for further processing.
As is known to filter designers, not only must an amplitude passband be accurately designed in the filter, but care must be given to provide a uniform phase response for the signal processing components. When a pulse signal or an analog signal having a complex wave form is processed, an equal time delay must be given to all of the components in the signal in order to avoid significant distortion to the signal processed by the filter. In an ideal design, the signal produced by the network component should only differ in amplitude from the input signal. In practice, however, a nonlinear time delay over frequency in the passband of interest, representing a non-linear phase response for the network component, is experienced. As a measure of the distortion added to a complex signal by the non-linear phase response, the first derivative of the phase response versus frequency is defined as the group delay for the device which constitutes a measure of the distortion produced by the non-linear phase response.
In digital signal processing the distortion may take the form shown in FIG. 1. Each transition of the digital signal produces an amplitude distortion because of the non-linear effect of the phase delay through a filter 10 and may produce inaccurate data decoding. In the case of analog signal processing, a signal which had been amplitude modulated and subject to a non-uniform group delay distortion creates envelope delay distortion which deteriorates the output signal.
The well known problem of group delay distortion is usually addressed by providing phase correction networks having a phase response which is complementary to the network filter phase response. When the filter and phase correction network are connected in tandem, the overall phase response will be compensated in the frequency band of interest. FIG. 2 represents a performance analysis of a typical 30 MHz low pass filter without group delay equalization. The amplitude response of the filter extends to substantially 30 MHz, and then begins to roll off. The group delay performance of the filter, however, is not a uniform response curve in the bandwidth of interest as is the foregoing amplitude response curve. The time delay as shown in FIG. 2, representing the phase response for the lowpass filter, is non-linear. The time delay increases from 15 nanoseconds at 100 KHz to 18 nanoseconds at 5 MHz, and then to a peak of 70 nanoseconds at 33 MHz. The non-linear group delay creates the distortion referred to earlier when a complex signal is processed by the filter, and may be corrected using a group delay equalizer specifically designed to provide a group delay curve which is a complement to the group delay curve of the filter. FIG. 3 illustrates the uncorrected time delay curve C(1) of the filter, and the correction curve C(2) group delay of the network, respectively. When the two curves are added together, a substantially flat, constant delay time response C(3) inside the frequency spectrum of interest, i.e., DC to 30 MHz is realized. However, a consequence of combining the two networks to derive a constant group delay results in overall (constant) delay time for the combined filter and equalizer network.
Conventional group delay equalization is implemented using an all pass filter having constant amplitude/frequency response and a phase-versus-frequency response which complements the response being corrected. While these networks are capable of producing a compensating phase response, the design is typically not trivial. A number of filter sections may be required, and because of interactions between sections, a tedious trial and error design process may be needed to obtain the necessary phase response for equalizing group delay.
The present invention provides for an all pass equalization network. The all pass equalization network includes a transformer which has a primary and secondary winding which effects the group delay function for the network. A bias inductor is connected to one of the ends of the primary and secondary winding, and to a common terminal for modifying the transformer coupling so that the group delay through the equalization network may be accurately controlled.
In a preferred embodiment of the invention, a second order all pass filter is constructed having a network Pole-zero (Q)  less than 1. One end of a primary winding, and one end of a secondary winding is connected, respectively, to the input and to the output of the filter, and the remaining ends of the primary and secondary winding are connected to each other. A coupling factor for the transformer is modified by a bias inductor connected to the primary and secondary remaining ends. A first and second capacitor are connected, respectively, between the input and output terminals of the all pass filter, and between the remaining end of the inductor and a common terminal.
The value of the bias inductor is selected to modify the slope of the group delay function of the all pass filter. By controlling the value of the bias inductor, the entire coupling factor for the transformer can be modified to a precisely theoretical design value.